Soil Acidity

ACIDITY MONITORING TOOLS

For environmental considerations in moderate and high rainfall areas it is important to monitor soil acidity and balance the acidifying effects of agriculture with lime. Acid soil problems rarely occur where rainfall is below 400mm per year.

To do this we recommend the following monitoring tools:A) LIMING DECISION TOOL

B) CAPITAL LIME APPLICATION (IF NEEDED)C) MAINTENANCE LIME RATE

A) LIMING DECISION TOOL

This monitoring tool involves using soil testing to assess pH and aluminium (Al) levels. Routine testing (0-10 cm depth) is advised as part of a regular soil fertility assessment program and deeper soil testing is recommended for problem paddocks where production is low due to poor plant growth.

Goals:

To maintain the topsoil pH at a level where all crop and pasture species can be grown; the preferred target pHCa is 5.5 which will permit lime movement below the depth of application. As a minimum, your target pHCa should be at least 5.0.

To maintain subsoils in a non-toxic pH condition so that all crop and pasture species can be grown (minimal Al present).

What you need to know before monitoring for soil acidity:

Soil pH (the measure of acidity) is a common soil measurement. Acidity affects plant growth by decreasing the availability of plant nutrients such as phosphorus and molybdenum and increasing the availability of some elements to toxic levels, particularly aluminium and manganese. Of these elements, aluminium toxicity is the most common. Soil testing for pH and Al is essential to identify the most likely limitations due to soil acidity.

Soil pH: Soil pH is most commonly measured in water (pHH2O or pHw) or a weak solution of calcium chloride (pHCaCl2 or pHCa). Soil testing laboratories often report both measures. The soil pHw considered to be closer to the pH which plant roots experience in the soil, but is subject to large seasonal variations. In comparison pHCa less affected by seasons and farmers can take samples at different times of year without affecting the diagnosis or interpretation. The pHw commonly 0.6 - 0.8 units higher than the pHCa although this difference gets less in saline soils (eg. 0.1-0.5 pH units higher in high salinity soils).

Aluminium: Soil clay minerals contain aluminium but it has no impact on plant productivity unless the pHCa falls below 4.8 (pHw less than 5.5). Toxic amounts of soluble aluminium become available and plant root growth can be severely reduced, affecting the plant's ability to take up water and nutrients. Some plants are more sensitive to aluminium than others (see Table 2). Aluminium can be measured in a number of different ways (see Table 3) and different laboratories offer different Al tests. The most common tests are:

Al measured as a percentage of the cation exchange capacity (Alex%) – the cation exchange capacity of soil is the sum of the levels of the cations of calcium, magnesium, sodium, potassium and aluminium. This percentage varies according to the salt content of the soil and thus Al % must be interpreted with the known salinity level (expressed in units of deci-siemens/m or dS/m).

Al measured in a weak solution of calcium chloride (AlCa or AlCaCl2 ) extracts most of the Al dissolved in the soil and which a plant root might encounter (expressed as mg/kg AlCa).

Al measured in potassium chloride (AlKCI) measures the amount of exchangeable Al that is potentially available to the plant during the growing season; it is not a particularly good indicator of total available toxic Al.

Steps for acid soil management using the liming decision tool

STEP 1. Select paddock and record details

If there have been any problems such as declining yields, particularly of acid-sensitive species such as canola, barley or acid-sensitive wheats, or unexplained establishment failures of lucerne, write these down. For suspected problem paddocks you should soil sample both 0-10 cm and 10-20 or 10-30 cm depths (and into the clay subsoil if you are very keen or suspect deep soil acidity).

Figure 1 illustrates the effect of subsoil acidity on acid-sensitive plants. The first plant (a) has grown well because the soil pH is only slightly acidic. Plant (b) has had to struggle through an acidic topsoil and is stunted by the time it gets to the subsoil but should persist. Plant (c) is growing very poorly due to the acidic topsoil and the acidic subsoil and would not be expected to persist. Plant (d) has had lime incorporated so is establishing OK but will run into trouble when the roots reach the acidic subsoil. This highlights the importance of knowing our subsoil pH before incurring the costs of crop and pasture establishment.

Soil sampling of both the o-10 cm and the 10-20cm depths will allow you to assess if you have a soil acidity limitation. Figure 2 below shows that the soil on the left has no sub soil acidity limitation but the soil on the right is acid at both depths.

Figure 2: Sampling below 10 cm depth as well as the topsoil will identify if there is an acidity problem in sub-surface layers (reproduced from NSW Agriculture Agfact AC 19 'Soil acidity and liming, 1996).

Equipment needed

soil sampling tool (or make one with a clean section of metal pipe which is 2 - 5 cm in diameter)

bags

waterproof marker

buckets to put soil samples into

How many samples do I need to take?

For 0-10 and 10-30 cm samples, at least 20 samples per soil type or paddock are needed. Put the 0-10 and 10-30 cm samples in separate buckets as you sample them.

If sampling into the clay B horizon, a minimum of 6 cores are needed. You will need an auger to sample these – do not use a spade. Put these samples in another bucket.

How do I collect the samples?

Sample from representative areas in the paddock – avoiding headlands, stock camps, stock tracks, fencelines, around trees, etc. If the paddock contains more than one soil type, choose the main soil type or take two lots of samples – depending upon how you intend to make the management decision if liming is recommended.

Do not sample paddocks for at least six weeks after fertiliser, lime or gypsum has been applied.

For paddocks less than 20 ha in size sample in a zig-zag pattern taking a sample every 50-100 metres. For larger paddocks sample in a diagonal transect across the paddock.

Once you have finished sampling, mix samples within each bucket well. Take one cupful from each bucket, put the soil into the plastic bag. Make sure the samples are labelled correctly (paddock name, date, depth of soil sample) with a waterproof marker.

Choose the soil testing option as suggested by the soil-testing laboratory. For 0-10 cm samples you should get a comprehensive soil fertility analysis. For 10-30 cm or subsoil samples, only pH, EC, aluminium and sodicity tests are required. Try to send samples to the laboratory within 24 hours.

STEP 3. Interpret soil test results.

You can use the soil testing laboratory recommendation, and/or interpret the soil test results for yourself, or get an agronomist to interpret them for you.

Match your soil test results to Table 1 to see the likely effects of your pH on plant growth and how sensitive your crop or pasture is to acidity (Table 2, next page).

Table 1: Major effects of different pH levels on plants.

Soil pHCa

Common effect on plant growth

6.5 – close to neutral

Optimum for many acid-sensitive plants. Some trace elements may become unavailable.

5.5 – slightly acid

Optimal balance of major nutrients and trace elements available for plant uptake.

5.0 – moderately acid

Below pH 4.8 aluminium (Al) can become toxic to plants, depending upon soil type. Phosphorus combines with Al and may be less available to plants.

Use Table 3 to assess your the aluminium soil test reading for plant tolerance (i.e, highly sensitive, sensitive, tolerant or highly tolerant) and then use Table 2 to assess whether the plants you are growing are likely to cope with the aluminium concentration in your soil.

Table 3: Critical Al concentrations for three different Al tests that will reduce plant yields.

Al tolerance of plants

Al ex% Low salinity (<0.07 ds/m)

Al ex% Med salinity (0.07-0.23 ds/m)

Al ex% High salinity (>0.23 ds/m)

Al ex% (mg/kg)

Al ex% (mg/kg)

Highly sensitive

9-16

2-8

0.5-2

0.5-2

15-30

Sensitive

16-21

8-12

2-6

2-4

30-50

Tolerant

21-32

12-21

6-10

4-8

50-100

Highly tolerant

32-43

21-30

10-16

8-13.5

>100

Note: Al can be measured in CaC12 , KCl or expressed as a % of cation exchange capacity – check your soil test report for which test was used before using this table.

If the plant you are considering growing is unlikely to cope with the aluminium concentration of your soil, then you will need to consider liming, and/or undertake deeper soil testing to determine if acidity is only in the topsoil.

STEP 4.Make decisions about acidic soil management options.

Figure 3 (see next page) can be used to assist you to make an informed long-term decision about what you need to do to maintain your soil at a pH at which all desired crop/pasture species can be grown.

There may be some cases however (where subsoil pH is extremely low and Al concentrations are very high) where liming is unlikely to be economic. In these cases land-use change (planting acid-tolerant perennials) may be the most appropriate strategy, both environmentally and economically.

Figure 3: Options for acidic soil management and liming

Use-acid tolerant species. Careful decisions based on economics. If acidity is throughout the whole profile, land-use change could be the best option. If subsoil acidity is not deep, consider liming to pH 5.5 to ameliorate subsoil.

It is unlikely to have a topsoil with a pHCa of 5.2-5.5 (slightly acid) and a subsoil with a pHCa < 4.2 (strongly / extremely acid)

pHCa 4.2-5.2

Can only use acid-tolerant species. Careful decisions based on economics.

If economic, lime to pH 5.5 and maintain pH at 5.5 to encourage lime movement to subsoil over time.

No acidity problem –monitor topsoil pH every 5 years to maintain or keep at pH 5.2-5.5

Maintenance lime

No acidity limitation: monitor topsoil pH every 5-10 years.

* Capital lime application: If you have a sub soil acidity problem, you need to apply higher amounts of lime than required just to fix surface soil acidity. Lime only moves below the depth of incorporation very slowly (can take decades). The higher the lime application to the surface soil, the greater the movement of lime below this layer will be. Capital lime application is the larger amount of lime required than that amount needed to fix the surface soil acidity.

B) CAPITAL LIME APPLICATION (IF NEEDED)

Goal : Capital lime application to increase the pH to 5.5 (CaCl2)

Steps to calculate capital lime application*:

* Capital lime application: If you have a sub soil acidity problem, you need to apply higher amounts of lime than required just to fix surface soil acidity. Lime only moves below the depth of incorporation very slowly (can take decades). The higher the lime application to the surface soil, the greater the movement of lime below this layer will be. Capital lime application is the larger amount of lime required than that amount needed to fix the surface soil acidity.

Where the topsoil is strongly acidic you should also consider sampling the 10-30 cm depth and possibly the deeper subsoil. Deep acidity problems can markedly affect the wisdom of a decision to apply a capital lime application. (see Figure 1).

STEP 2.Calculate the lime requirement based on your soil test results and the desired target pH.

Soil test reports will often give a lime requirement figure – make sure you understand the basis on which this calculation is made – is it to raise the soil pH to pH 5.2 or pH 5.5 for example? You can also calculate your own lime requirement based on the information outlined below:

a) Soil test and subtract your soil pH result from 5.5 (if you are liming to a target pHca of 5.5).

Let's assume a soil test result of 4.2, thus 5.5-4.2 = 1.3.

b) Divide this number by a conversion factor for particular soil types: 0.26 for clay, 0.37 for clay loam, 0.47 for sandy clay loam and 0.57 for sandy loam.

c) The lime rates we have calculated above assume a pure limestone product. To make a decision about which lime product to use you need to know the price of those available and their effective neutralising values (ENV) (this depends on their purity and fineness).As an example let's assume that there are two lime products available:

Lime A has an ENV of 95 and costs $60/t spread and Lime B has an ENV of 70 and costs $50/t spread.

The calculation is total cost of lime per tonne spread/ENV;Lime A costs $60/95 = $0.63 per unit of ENV and Lime B costs $50/70 = $0.71 per unit of ENV.

Lime A is cheaper. To calculate the amount of Lime A you need, assuming you wish to apply 3.25 t/ha from the example we outlined above, then you will need to apply (3.25 x 100)/95 = 3.42 t/ha of Lime A. This accounts for the 5% of Lime A that does not neutralise soil acidity – the 3.25 t/ha figure assumed a lime quality of 100% ENV.

C) MAINTENANCE LIME APPLICATION RATE

Goal: To balance the acidifying effects of agricultural production by applying lime to maintain topsoil and subsoil pH and Al at levels where all desired species can be grown.

What you need to know about soils becoming more acidic under agriculture:

Agricultural production makes soils more acidic. For environmentally sustainable farming systems, it is important to apply maintenance dressings of lime to balance the acidification caused by farming. Maintenance dressings only need to be applied every 5-10 years as part of a liming program and should be considered as a fixed cost of the production system. These maintenance dressings, to balance the ongoing soil acidification of agriculture, are in addition to the capital lime requirement that might be needed to increase the pHca to 5.5.

Nitrate leaching occurs when water leaks below the root zone taking soluble N with it (the nitrate form of N). It is relatively easy to calculate the effects of acidification from removal of produce (grain, hay, animals), but more difficult to determine N leaching and fertiliser N losses – to do this we will make some 'best bet' assumptions about long-term nitrate leaching losses.

Steps to calculate the maintenance lime application rate:

STEP 1. Calculate the amount of lime required to balance acidification due to product removal.

Estimate (or obtain from good paddock records) the long-term (10 year) average paddock crop yields and stocking rates of the paddock of interest.

* Use Tables 4 and 5 to calculate the lime required to replace that removed in produce.

AMost of the lime requirement, 24 kg, is due to net transfer of urine and dung to stock camp areas.BFor every DSE/ha over 10 DSE/ha add a further lime requirement of 3 kg lime/ha/DSE.

If your paddock records with respect to stock movements are poor just assume the per ha figures in Table 5 for the years when the paddock was in pasture.In an example, let's assume that over a 10 year period two wheat crops have been grown

at an average yield of 3 t/ha, one lupin crop at 1 t/ha and stock have grazed the paddock for the remaining seven years. In one very good year 4 t/ha of mixed clover/grass hay was also cut. The approximate calculation, using Tables 4 and 5, would be (2 x 3 x 9 + 1 x 20 + 7 x 30 + 4 x 30)/10 = 40 kg lime/ha/year lime requirement.

STEP 2. Calculate the assumed amount of acidification associated with use of fertiliser N.

If you have applied N fertiliser you will need to work out from paddock records how much you have applied over a 10 year period.

A This is somewhat simplified because the actual leaching potential will depend upon when the N was applied and whether wet conditions follow and enhanced the leaching potential.

In the example, let's assume that for the two wheat crops, a topdressing of 50 kg N/haurea was given to one and the other received 40 kg/ha MAP. The calculation is therefore(50 x 2 + 40 x 6)/10 = 34 kg lime/ha/year, averaged over the 10 years.

STEP 3. Estimate the amount of nitrate leaching from legume N inputs.

Use the estimates in Table 7 below to estimate likely nitrate leaching.

Assume in our example that the average annual rainfall is less than 500 mm/year andthe soil type was a loam. The paddock was in either annual crop or pasture for the 10 year period and thus from Table 7 we assume that the lime requirement due to nitrate leaching is 75 kg lime/ha/year.

STEP 4. Add up the amounts of lime needed to balance the acidification from product removal, N fertiliser application and leached legume N to determine the total lime.

Apply lime approximately every 10 years to balance the calculated acidification rate.

40 + 34 + 75 = 149 kg lime/ha/year or 1.5 t/ha needs to be applied every 10 years to balance the soil acidification rate in this example.

STEP 5.Account for subsoil acidity impacts. Recent work has shown that the calculated lime rates required to maintain topsoil pHs are an under-estimate because some of the lime can move to the subsoil – encouraging news for long-term sustainability.

Multiply the calculated total lime requirement in Step 4 above by 1.3 to account for maintaining subsoil pH. 149 kg x 1.3 = 194 kg lime/ha/year or approximately 2 t/ha every 10 years to fully account for maintenance lime rates.

Breaches of the Water Act.

Breaches of the Water Act may occur if appropriate approval has not been sought from the local Water Authority when soil sampling, installing piezometers or neutron probe access tubes. In general, if any soil sample, piezometer, soil moisture tube, is greater than 3 metres deep, or if it intercepts the groundwater, it may have to be registered with the relevant water authority. Individual water authorities could have different approaches to registration. Breaches of the Water Act are an offence that involves a financial penalty.

Registration requires a series of steps that generally involve:

contacting the local water authority and applying for installation licence

this licence will require maps of locations, depth, purpose and information regarding near by infrastructure. A dial before you dig, that locates underground infrastructure, power etc may also be required. If the person has never registered a bore, or a hole in the ground so to speak, then it is strongly advised they contact the water authority to determine the steps involved.

the cost to register - roughly $400 for first hole and an additional $50 for second etc, but this may vary with the water authority.

The registration process exists for several reasons. For example:

in an attempt to prevent pollution by aquifer leakage, this is why soil sampling is included in the registration process if groundwater is intercepted.

to provide an identification number and location of groundwater bores, and generate funds to run the groundwater data bases.

to ensure no illegal groundwater extraction.

The registration process also means that only a registered driller can install the holes or collect the soil samples. If you unsure of the groundwater depth in your region, then using a registered driller removes any risk.

There are generally no exceptions to the rule, however there are variations in how the Act is governed by water authorities, so contact with water authorities is essential. Also, as a low cost risk management strategy, it is recommended to contact the dial-before-you-dig hotline (phone 1100 or www.dialbeforeyoudig.com.au) and ascertain the location of any infrastructure (power, telecommunications, gas etc.).

Reference list:

A comprehensive decision support guide ('Acid Soil Action, investment for your soil now and in the future – a practical decision support guide to assess the problem and manage the risk') has also been developed by Carole Hollier (0260 304500; $40 each).

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